In French Pat. No. 2,385,252 for a "Static AC-AC thyristor converter for a self-driven synchronous motor", a means is described for reducing the detrimental effect on the motor torque of switching in the bridge which feeds the synchronous machine, which bridge is herein referred to as the machine bridge. Since said detrimental effect is caused during low-frequency operation, i.e. at a frequency below the minimum at which the machine bridge can operate in natural switching mode since the electromotive force of the machine is too weak, and when operating in the forced switching mode which is sometimes referred to as sequential or clocked mode, by controlling the thyristor bridge connected to the mains using thyristor firing angles of .alpha. in such a way that they are at an angle herein referred to as the full inverter firing angle. The thyristors of the machine bridge are then switched to the zero current position. The consequence of this operation is that the motor torque drops to zero each time the machine bridge switches when operating at very low speeds.
In the above-mentioned patent, said disadvantage is remedied by adding an auxiliary connection which connects the centre of a star-connected stator winding of the synchronous machine either to the centre of a star-connected secondary winding of the transformer which energizes the mains bridge or else, in the case where the mains bridge is constituted by two complete series-connected bridges, to the point where the two bridges are series-connected; and by controlling the upper half bridge separately from the lower half bridge or else by controlling the two series-connected bridges separately.
This makes it possible to maintain current in the non-switched phase of the motor, the current flowing along one or other of the two connections which connect the mains bridge to the machine bridge and along the auxiliary connection. This makes it possible to reduce fluctuation in torque.
The present invention provides another method of reducing torque fluctuations.